1. Field of the Invention
This invention is concerned with devices, such as adjustable mirrors, smart windows, optical attenuators and displays, for controlling the reflectance and/or transmission of electromagnetic radiation.
2. Description of the Related Art
Sunlight transmitted through windows in buildings and transportation vehicles can generate heat (via the greenhouse effect) that creates an uncomfortable environment and increases air conditioning requirements and costs. Current approaches to providing xe2x80x9csmart windowsxe2x80x9d with adjustable transmission for use in various sunlight conditions involve the use of light absorbing materials. Such approaches are only partially effective since the window itself is heated so that heat is transferred into the interior by convection. In addition, these devices, such as electrochromic devices, are relatively expensive and exhibit limited durability and cycle life. Certain liquid crystal-based window systems switch between transmissive and opaque/scattering states, but these systems require substantial voltages to maintain the transparent state. There is an important need for an inexpensive, durable, low-voltage smart window with variable reflectivity. Reflecting the light, rather than absorbing it, is the most efficient means for avoiding inside heating. Devices for effectively controlling transmission of light are also needed for a variety of other applications. For example, an effective means for controlling light transmission over a wide dynamic range is needed to permit use of inexpensive arc lamps as light sources for projection displays.
Bright light from headlamps on following vehicles reflected in automobile rear and side view mirrors is annoying to drivers and creates a safety hazard by impairing driver vision. Currently available automatically dimming mirrors rely on electrochromic reactions to produce electrolyte species that absorb light that would otherwise be reflected from a static mirror. Such devices do not provide close control over the amount of reflected light, and are expensive to fabricate since a very constant inter-electrode spacing (i.e., cell gap) is required to provide uniform dimming. Image sharpness is also reduced for electrochromic mirror devices since the reflected light must pass through the electrolyte (twice). There is an important need for an inexpensive adjustable mirror device that provides close control of reflected light with minimal image distortion.
Some earlier workers attempted to exploit reversible electrodeposition of a metal for light modulation, primarily for display applications [see for example, J. Mantell and S. Zaromb, J. Electrochem. Soc. 109, 992 (1962) and J. P. Ziegler and B. M. Howard., Solar Eng. Mater. Solar Cells 39, 317, (1995)]. In these cases, metal, typically silver or bismuth, was reversibly electrodeposited onto a transparent working electrode, usually indium tin oxide (ITO), from a thin layer of electrolyte sandwiched between the working electrode and a counter electrode. Both water and organic liquids (e.g., dimethylsulfoxide or dimethylformamide) were employed as solvents. The deposits obtained on the transparent electrode presented a rough and black, gray, or sometimes colored appearance (typical of finely-divided metals) and were used to enhance light absorption by display elements. Pigments were often added to the electrolyte to provide a white background for improved contrast. An auxiliary counter electrode reaction (e.g., halide ion oxidation) was typically employed so as to provide a voltage threshold (which is needed for matrix addressing) and/or to avoid metal deposition on a transmissive counter electrode (which would offset the light modulation provided by metal deposition on the working electrode). Such auxiliary reactions introduced chemistry-related instabilities during long term operation and led to deposit self erasure on open circuit via chemical dissolution of the metal deposit. Nonetheless, the key drawback of reversible metal electrodeposition for display applications was the relatively slow response for attaining adequate light blocking.
A reversible electrochemical mirror (REM) device permitting efficient and precise control over the reflection/transmission of visible light and other electromagnetic radiation is described in U.S. Pat. Nos. 5,903,382, 5,923,456, 6,111,685 and 6,166,847 to Tench et al., which are assigned to the same assignee as the present application. In this device, an electrolyte containing ions of an electrodepositable metal is sandwiched between a mirror electrode and a counter electrode, at least one of which is substantially transparent to the radiation. A typical transparent mirror electrode is indium tin oxide (ITO) or fluorine doped tin oxide (FTO) deposited on a transparent glass (or plastic) pane which serves as the substrate. Application of a voltage causes the electrodepositable metal, e.g., silver, to be deposited as a mirror on the mirror electrode while an equal amount of the same metal is dissolved from the counter electrode. When the voltage polarity is switched, the overall process is reversed so that the electrodeposited mirror metal is at least partially dissolved from the mirror electrode. A thin surface modification layer of noble metal, e.g., 15-30 xc3x85 of platinum, on the transparent conductor is usually required to improve nucleation so that a mirror deposit is obtained. The thickness of mirror metal layer present on the mirror electrode determines the reflectance of the device for radiation, which can be varied over a wide range.
The REM technology can be used to provide control of either light reflectance or transmission, or both. A transmissive REM device suitable for smart window applications utilizes a noble metal counter electrode that is locally distributed, as a grid for example, on a transparent substrate, e.g., glass or plastic, so that mirror metal deposited thereon does not appreciably increase light blockage. In this case, high light transmission is provided by a locally distributed counter electrode of relatively small cross-sectional area and the device reflectance/transmission is adjusted via the thickness of mirror metal on the mirror electrode. As described in U.S. Pat. No. 6,166,847 to Tench et al., which is assigned to the same assignee as the present application, such a transmissive counter electrode is not required for reflective REM devices used for adjustable mirror applications. An electrolytic solution providing the inherent stability, high deposit quality, complete deposit erasure, long cycle life, and reasonably fast switching needed for most practical applications is described in U.S. patent application Ser. No. 09/619,127, filed Jul. 18, 2000, to Tench et al., which is assigned to the same assignee as the present invention. This solution is typically comprised of 1.5 M AgI and 2.0 M LiBr in a gamma-butyrolactone (GBL) solvent, and may also contain highly dispersed silica (HDS) added to produce a gelled electrolyte and/or dispersed carbon added to blacken the electrolyte so as to reduce background light reflection.
Switching speed for REM devices is presently limited by the solubilities and transport rates of electrodepositable metal ions in the conventional solvent-based electrolytes available. For transmissive REM devices employing a localized distributed electrode (e.g., a grid), the maximum cell current is severely limited by the relatively small area of the counter electrode so that enhanced electrolyte current carrying capability would be particularly advantages. In addition, electrolytes having the high ionic strength and low ion pairing needed for fast switching also have relatively low electrical resistivity so that a wider cell gap is needed to provide sufficient electrolyte resistance for uniform mirror switching. Note that good mirror uniformity is obtained when the electrolyte resistance is large compared to the electrode sheet resistances. For large-area devices, the width of the cell gap required to provide an acceptable electrolyte resistance can greatly increase the electrolyte volume, and thus the cost and weight of the device. An electrolyte providing enhanced current carrying capability coupled with relatively high resistivity would greatly enhance performance of REM devices and broaden their potential applications. Faster switching speed is particularly important for display optical attenuators, and might also enable use of reversible electrodeposition devices for some types of displays.
A possible approach for increasing the allowable current densities for reversible electrodeposition reactions is to utilize ionic liquid electrolytes, which are comprised of mixtures of a metallic salt and an ionic organic compound that are liquid at or near ambient temperatures. Since no solvent is required, the concentration of electrodepositable metal ions in such electrolytes could conceivably be very high (around 6 M compared to 1 to 3 M for most solvent systems). Ionic liquids exhibiting reasonably high electrical conductivity generally involve heterocyclic organic cations having one or two nitrogen atoms in a five- or six-member ring structure. The most widely studied ionic liquids have been the chloroaluminate salts with AlnCl3n+1xe2x88x92 cations (where n is typically between 1 and 2) and 1-ethyl-3-methylimidazolium (EMI+) or N-butylpyridinium (BuPy+) cations, from which aluminum metal can be electrodeposited.
From the teachings of the prior art literature, metal deposition from ionic liquids would appear to be unsuitable for reversible electrodeposition light modulation devices, especially those requiring high reflectance. For example, aluminum deposits obtained from neat EMI+ chloroaluminate systems are reported to be powdery and nonadherent [Q. Liao, W. R. Pitner, G. Stewart, C. L. Hussey and G. R. Stafford, J. Electrochem. Soc. 144, 936 (1997)], or to range from dull gray to black depending on the deposition voltage [R. T. Carlin, W. Crawford and M. Bersch, J. Electrochem. Soc. 139, 2720 (1992)]. Better quality aluminum deposits (described as xe2x80x9csilver whitexe2x80x9d) can apparently be obtained from the BuPy+ chloroaluminate system [M. R. Ali, A. Nishikata and T. Tsuru, Indian J. Chem. Technol. 6, 317 (1999)] but the maximum current density for this three-electron reaction was only about 3 mA/cm2, which is equivalent in terms of deposited metal atoms to only 1 mA/cm2 compared to one-electron silver deposition. A switching current density for REM devices of 4 mA/cm2 is provided by the silver halide electrolyte with a GBL solvent described in U.S. patent application Ser. No. 09/619,127, filed Jul. 18, 2000, to Tench et al., which is assigned to the same assignee as the present application. The prior art literature further teaches that ionic liquid electrolytes provide shiny aluminum deposits only when mixed with conventional organic solvents (benzene or toluene, for example) and only over a very limited current density range [F. H. Hurley and T. P. Wier, Jr., J. Electrochem. Soc. 98, 207 (1951) and Q. Liao, W. R. Pitner, G. Stewart, C. L. Hussey and G. R. Stafford, J. Electrochem. Soc. 144, 936 (1997)], making them unsuitable for reversible electrodeposition optical modulation applications. In any case, silver is inherently more reflective for visible light than other metals and is preferred for REM light modulation devices.
The prior art literature further teaches that metals other than aluminum do not readily form ionic liquids suitable for reversible electrodeposition devices. For example, the maximum current density for deposition of cobalt from a chloride ionic liquid containing molar ratios of 3.2 Co(II), 5.4 BuPy+ and 0.1 Cr(II) was less than 1 mA/cm2 at 110xc2x0 C. [M. R. Ali and T. Tsuru, Indian J. Chem. Technol. 8, 44 (2001)]. Consequently, prior art work has focused on utilizing chloroaluminate ionic liquids as solvents for the ions of the electrodeposited metal. For example, electrodeposition of copper from low concentrations of Cu(I) ions (20 mM or less) in the EMI+ chloroaluminate solvent system has been investigated [Q. Zhu and C. L. Hussey, J. Electrochem. Soc. 148, C395 (2001); and J. J. Lee, B. Miller, X. Shi, R. Kalish and K. A. Wheeler, J. Electrochem. Soc. 148, C183 (2001)]. This solvent system has also been used to study electrodeposition of lanthanum at the solubility limit (only 45 mM) [T. Tsuda, T. Nohira and Y. Ito, Electrochim. Acta 46, 1891 (2001)]. Cobalt has been electrodeposited (two-electron process) from a 0.24 M solution of Co(II) ions in chloroaluminate BuPy+ solvent but the maximum current density was only about 2 mA/cm2 [M. R. Ali, A. Nishikata and T. Tsuru, Electrochim. Acta 42, 1819 (1997)]. Maximum current for deposition of copper from the alternative EMI+ chlorozincate solvent containing 0.3 M Cu(I) was about 3 mA/cm2 at 80xc2x0 C. [P. Y. Chen, M. C. Lin and I. W. Sun, J. Electrochem. Soc. 147, 3350 (2000)]. These examples also illustrate that the approach of using an ionic liquid as a solvent greatly reduces the benefit that could otherwise be provided since the concentration of electrodeposited metal ions is thereby reduced and is limited by solubility considerations, as is the case with conventional solvents.
Ionic liquid electrodeposition of silver, the preferred REM mirror metal, has apparently only been studied at ambient temperatures for small concentrations of silver ion (25 mM) dissolved in liquid EMI+ tetrafluoroborate [Y. Katayama, S. Dan, T. Miura and T. Kishi, J. Electrochem. Soc. 148, C102 (2001)] and EMI+ chloroaluminate systems [Q. Zhu, C. L. Hussey and G. R. Stafford, J. Electrochem. Soc. 148, C88 (2001)]. Steady-state silver deposition currents for the unstirred liquids were less than 0.4 mA/cm2 in both systems. For the tetrafluoroborate system, xe2x80x9ca silver-white deposit without brightnessxe2x80x9d was obtained. Electrodeposition of silver and several other metals from fused mixtures of the metallic chlorides and ethyl pyridinium bromide at 135xc2x0 C. has been reported [F. H. Hurley and T. P. Wier, Jr., J. Electrochem. Soc. 98, 203 (1951)] but this temperature would be unsuitable for most optical modulation devices and the quality of the electrodeposits obtained was not stated.
The present invention is a reversible electrodeposition optical modulation device employing an ionic liquid electrolyte, which is comprised of a mixture of an ionic organic compound and the salt of an electrodepositable metal. For high concentrations of electrodepositable metal ions in the ionic liquid electrolyte, the effects of mass transport limitations are minimized and high currents for metal deposition and dissolution can be sustained. It is generally preferable to maximize the concentration of electrodepositable metal ions by utilizing an ionic liquid not containing substantial quantities of other metal ions. In this case, the concentration of electrodepositable metal ions and the diffusion-limited deposition/dissolution currents can be as much as four times higher than those for a typical solvent-based electrolyte. In addition, the conductivities of ionic liquids are usually at least an order of magnitude lower than those of electrolytes employing a solvent, which enhances the uniformity of the electrodeposit obtained for a given set of conditions. Thus, the ionic liquid electrolyte of this invention also enables uniform electrodeposition (and electrodissolution) with thinner electrolyte layers, which can significantly reduce the cost of the electrolyte. Ionic liquids also offer a wide operating temperature range and have practically no vapor pressure, which facilitates device sealing and operation at elevated temperatures. At ambient temperatures, ionic liquids tend to be very viscous and adhesive, which expands cell assembly options and minimizes electrolyte splattering during cell breakage.
One embodiment of the present invention is a reversible electrochemical mirror (REM) device employing an ionic liquid electrolyte (containing electrodepositable metal ions) in contact with a transparent mirror-forming electrode and a counter electrode. The mirror electrode is typically comprised of a thin surface modification layer of noble metal (e.g., platinum) on a layer of a transparent conducting oxide (e.g., indium tin oxide) on a glass or plastic substrate. The noble metal layer enhances nucleation so that mirror electrodeposits are obtained. Generally, the counter electrode is a sheet or layer of the electrodepositable mirror metal for devices that are designed to control radiation reflection, and is a locally distributed inert metal electrode for devices that also transmit radiation. The device reflectance is determined by the thickness of the mirror metal layer on the mirror electrode, which can be adjusted by applying a voltage of the appropriate polarity to cause mirror metal electrodeposition or dissolution, while the reverse process occurs at the counter electrode. Applications for REM devices include automotive rear and side view mirrors with adjustable reflectivity, attenuators for display brightness control, and smart windows for use in transportation vehicles and buildings.
Another embodiment of the present invention is a reversible electrodeposition display device employing an ionic liquid electrolyte. A typical device of this type involves reversible metal electrodeposition on electrically isolated and separately addressable electrodes, which serve as display elements (e.g., pixels or alpha-numeric segments). Use of an ionic liquid electrolyte for display devices provides both faster switching and enhanced electrolyte resistance, which enhances switching uniformity and suppresses cross-talk between display elements. Poorly-reflecting electrodeposits may be used on display elements to block or absorb light, or an inert surface modification layer may be used to provide mirror deposits that reflect light. Reflective elements may be viewed directly or used for projection displays.
Suitable ionic liquids for reversible electrodeposition optical modulation devices include those comprised of electrodepositable metal ions, halide (or pseudohalide) anions, and heterocyclic-organic cations having one or two nitrogen atoms in a five- or six-member ring structure. Suitable organic cations include N-methylpyrrolidiinum (MP+), pyrrolidinium (P+), 1-ethylimidazolium (EI+), 1-ethyl-3-methylimidazolium (EMI+), 2-methyl-1-pyrrolinium (2M1P+) and N-butylpyridinium (BuPy+). Silver is a preferred metal since it provides high reflectivity (needed for REM devices) and is electrodeposited in a one-electron process (enhances switching speed). A variety of other metals can be used, including copper (which can involve a one-electron process), tin, zinc and alloys thereof. Ionic liquids made with these metals tend to be substantially transparent to visible light. Preferred anions are halides (fluoride, chloride, bromide and iodide) and pseudohalides (cyanide and thiocyanate), which provide the metal complexing needed for ionic liquid formation and more controlled metal deposition. Other anions whose compounds with organic cations form ionic liquids with electrodepositable metal salts may also be used. The ionic liquid electrolyte of the present invention may be rendered more viscous, semi-solid or solid by addition of organic or inorganic gelling agents. Inorganic or organic materials, including suspended carbon and dissolved dyes, may be added to the electrolyte to impart a desired color or to reduce background reflection. Some ionic liquids tend to slowly crystallize at room temperature to form opaque solids but this can be avoided by use of low-symmetry cations, mixtures of different cations, or mixed anions to introduce chemical asymmetry.
Halide ionic liquid electrolytes containing pyrrolidinium and N-methylpyrrolidinium cations have been found to provide particularly high current carrying capability for reversible electrodeposition of a variety of metals, including silver, copper, tin, zinc, and silver-palladium alloys. Good mirror formation in REM devices was observed in all cases. These cations (in halide systems) also provided moderately high electrical resistivity. For pyrrolidinium-based silver halide ionic liquids, the current carrying capability for reversible electrodeposition was usually greater than 10 mA/cm2 and the resistivity varied from 300 to 1200 ohm-cm, depending primarily on the halides used. Good mirror uniformity was obtained with ionic liquids incorporating these cations in REM cells having even small electrode spacings (0.2 mm). Systems incorporating mixtures of the two cations and/or different halides (chloride, bromide and iodide) did not crystallize at ambient temperatures and were apparently stable over the temperature range from at least xe2x88x9220xc2x0 C. to 150xc2x0 C. Finely-divided carbon suspended in a pyrrolidinium-based electrolyte provided a REM device with low reflectivity and did not appear to otherwise affect the device performance. A semi-solid gel electrolyte was formed by addition of highly dispersed silica (HDS). A variety of pyrrolidinium derivatives might also be used as cations for reversible electrodeposition ionic liquid electrolytes.
Optimum switching of reversible electrodeposition devices employing ionic liquid electrolytes is attained by automatically adjusting the drive voltage (depending on the current) to compensate for the resistive loss (IR drop) in the electrolyte. Fast switching is provided by measuring the current and increasing the drive voltage by the magnitude of the voltage drop in the electrolyte. Device degradation due to excessive electrode voltage is avoided by decreasing the applied voltage as the current and the electrolyte IR drop decrease during the later stages of deposit erasure.
A REM device with an ionic liquid electrolyte comprised of 28.5 mole % pyrrolidinium chloride, 28.5 mole % N-methylpyrrolidinium chloride and 43 mole % silver chloride was cycled with an IR-compensated applied voltage at an average current of 8 mA/cm2 for 35,000 cycles (between 0 and 500 xc3x85 silver mirror) with no change in mirror quality or switching performance. This is double the maximum current density for the best GBL electrolyte (1.5 M AgI+2.0 M LiBr) used in REM cells.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.